1. Technical Field
The present invention is directed to acoustic inspection of a casing cemented in a borehole. More particularly, the present invention is directed to acoustic inspection of the casing for determining specific properties relating to the casing and the surrounding materials, while simultaneously compensating for variations in the acoustic pulse and acoustic attenuation caused by mud filling the borehole.
2. Background Information
In general, once a well has reached a desired depth, the borehole is cased with cement being injected into the annular space between the casing and the wall of the borehole to prevent fluid communication between various geological strata. In order to determine whether such unwanted communication nevertheless exists, measurements may be performed downhole by means of a logging tool. These measurements determine the quality of the bond between the cement and the casing. Additionally, casing thickness can be derived from these measurements.
It has long been the practice to use acoustic waves for performing such measurements. Such techniques have relied on measurements which are averages in the circumferential and/or longitudinal direction of the casing, and consequently cannot identify localized phenomena such as longitudinal hydraulic communication paths. A complete presentation of prior art techniques can be found in U.S. Pat. No. 4,255,798 to Havira, assigned to the same assignees as the present invention and herein incorporated by reference.
Of the techniques that have sought to improve the vertical and radial resolution in such inspection, the technique described in the above-mentioned Havira patent turns out to have been an extremely important breakthrough. This technique consists of emitting an acoustic pulse over a radial sector of the casing, with the pulse being constituted by acoustic waves at frequencies selected to cause resonance to appear across the thickness of the casing; in determining the energy present in a reverberation segment of the reflected signal; and in characterizing from the energy the quality of the bond of the cement behind the radial sector of the casing. The reverberation segment under consideration is selected so as to be substantially representative of acoustic reverberation between the walls of the casing. Rapid damping of the resonance, i.e., low energy, indicates cement behind the casing, whereas slow damping, i.e., high energy, indicates an absence of cement.
A logging tool using the Havira technique is described in a commercial brochure entitled "Cement Evaluation Tool" published by Schlumberger in June 1983, incorporated herein by reference. The sonde, preferably centered within the casing, includes eight transducers distributed helically at 45.degree. intervals, thereby obtaining good coverage around the periphery of the casing. Acoustic pulses are fired sequentially. They are likewise received sequentially, analyzed and transmitted to the surface where they are processed.
In addition, a ninth transducer, commonly referred to as the "reference" transducer, points along the axis of the casing towards a reflecting wall which is plane and disposed at a fixed distance from the reference transducer. The reflection signal detected by the ninth transducer is used to determine in situ the propagation time through the borehole fluid (mud), i.e. the time interval between emission and reception of the acoustic wave. The wave's propagation velocity through the mud is deduced therefrom. Given the propagation velocity of the wave, it is possible to determine the apparent radius of the casing for each of the eight transducers. It is particularly advantageous to obtain this radius since it makes it possible, in particular, to detect any possible deformation of the casing and to monitor the centering of the sonde inside the casing in order to obtain an indication of the validity and the quality of the measurements as performed.
In this technique, a portion of the reflected acoustic signal, representative of acoustic reflections between the walls of the casing, is analyzed. A signal QC.sub.i is derived therefrom, representative of the quality of the bond of the cement with the casing, on the basis of the energy W2 measured in a reverberation segment of the reflected signal S.
The changes in amplitude of signal S as a function of time, as received by a transducer, are shown in FIG. 2. The effects due to mud are taken into account by normalizing the measured energy W2 relative to the peak amplitude signal W1. Nevertheless, it turns out that normalization does not give complete satisfaction since quantitative interpretation of the measurements shows up problems of divergence in the measurements.
Tests and experiments performed by Applicant have shown that these differences to a large extent can be attributed to a large extent to the fact that the conventional processing is sensitive to the properties of the mud, to the characteristics of the transducer and of its drive electronics.
The Applicant has observed that the normalization of the portion of the signal at energy W2 was being performed using a peak amplitude signal W1 which did not correspond to the same frequencies as those present in the portion of the signal corresponding to the energy W2.
In general, the energy W1 is not at the same frequency as the energy W2, even in water. The energy W1 is a measure of the energy maximum conveyed by a spectrum component which depends on the properties of the pulse, i.e. the characteristics of the transducer and its drive electronics, and on the attenuation by the mud, whereas the energy W2 includes energy only around the resonance frequency of the casing.